Conventionally, a transparent epoxy resin composition comprising an alicyclic epoxy resin, a curing agent and a curing catalyst has been widely used as an encapsulating composition for optical semiconductor elements. The epoxy resin composition is poured into a mold in which an optical semiconductor element is placed in a molding method such as a casting or transfer molding method, and cured to encapsulate the optical semiconductor element in a cured product (Japanese Patent NO. 3241338 and Japanese Patent Application Laid-Open No. Hei-7-25987).
In recent years, as brightness and output of blue-LEDs and white-LEDs have continued increasing, such problem that a LED element encapsulated with a conventional transparent epoxy resin discolors over time, i.e., yellowing, is caused by the blue light or the ultraviolet light which have short wavelengths. Further, a water absorption rate of a cured product obtained from the transparent epoxy resin composition is so high that the moisture resistance is poor.
Japanese Patent Application Laid-Open Nos. 2002-327126 and 2002-338833 disclose, as a coating and protective material for optical semiconductor elements, a resin composition which comprises an organic compound or silicone resin which has at least two carbon-carbon double bonds reactive with an SiH group; a silicon compound having at least two SiH groups; and a hydrosilylation catalyst.
However, a cured product obtained from the aforementioned silicone resin composition has such demerits that a surface of the cured product is tacky to collect dust and a light transmission is low. In order to solve the aforesaid problems, Japanese Patent Application Laid-Open Nos. 2002-314139 and 2002-314143 disclose a coating and protective material for optical semiconductor elements, which material contains a high hardness silicone resin. However, toughness and adhesiveness of the high hardness silicone resin are poor. Therefore, in a case of case-type light emitting semiconductor devices having a light emitting element placed in a ceramic or plastic housing, the silicone resin peels off from the ceramic or plastic housing to cause cracks during the housing filled with the silicone resin composition is subjected to a heat shock test at −40 degrees C and 120 degrees C.
The composition comprising an epoxy resin and a silicone resin was proposed in Japanese Patent Application Laid-Open No. Sho-52-107049 and Japanese Patent No. 3399652. However, the composition has such problems as poor adhesiveness and discoloration due to light degradation. Further, Japanese Patent Application Laid-Open No. 2004-238589 discloses a cured product obtained by curing a composition which comprises a siloxane compound having an epoxy group and/or an oxetanyl group and a silsesquioxane compound in the presence of a cation curing catalyst in order to improve strength of the resin and an UV resistance. However, the composition has a problem that the cured product thus obtained is corroded and discolored by oxonium ions which are derived from the cation curing catalyst. Japanese Patent Application Laid-Open No. 2005-263869 discloses a B-stage resin composition comprising a silsesquioxane compound and a hydrogenated epoxy resin. However, the composition has a problem such as a poor ultraviolet resistance. Japanese Patent Application Laid-Open No. 2004-99751 discloses a composition comprising an organopolysiloxane having a group derived from an isocyanuric acid and an epoxy resin, but the composition is not a transparent resin composition for encapsulating an optical semiconductor element.